Unwinding of plastic film in the presence of a plasma

ABSTRACT

Supplying gases towards the web as it separates from the drum in a plasma-enhanced chemical vapor deposition system reduces the sticking of the web to the drum, and thus prevents power supply dropouts. The gas supplied can form into a plasma that helps dissipate the static charge which builds onto the web as it rolls off of the drum. By reducing the arcing and power supply dropouts, the quality of the deposited layer formed in a deposition zone can be improved.

BACKGROUND OF THE INVENTION

The present invention relates to systems of coating a layer on a plasticfilm. The invention especially relates to the use of plasma-enhancedchemical vapor deposition (PECVD) to deposit a layer on a film thatrolls over a rotatable drum electrode.

An example of such a process is described in Felts, et al. U.S. Pat. No.5,224,441, which is incorporated herein by reference. In this reference,gases including a monomer (i.e., organosilicon), oxygen, and inert gases(i.e., helium) are introduced into an evacuated chamber. A glowdischarge plasma is formed between electrodes powered by the alternatingcurrent (AC) power supply. A web substrate moves over a drum electrodeand is treated by the plasma so that a silicon oxide layer forms on theweb.

A problem that can occur in the plasma-enhanced chemical vapordeposition involves power supply drop-outs. In power supply dropouts,the power supply turns off and must be re-started. While the powersupply is off, the web continues to roll over the drum electrode. Thiscan cause a reduction in the quality of the deposited layer on theplastic film during the time that the power supply is off.

It is desired to reduce the frequency of power supply dropouts and thusimprove the quality of the deposited layer on the film.

The inventors have found that some of the power supply dropouts arecaused by the plastic web material sticking to the drum. Static chargebuilds up on the web, causing it to stick to the drum. When the websticks to the drum, control over the level of tension on the web islost. The loss of tension can cause the web to separate from the drum.Good contact with the drum is needed for heat transfer and processstability while traveling through the process zone. Additionally, arcingand power supply dropouts can occur, when the web is eventually rippedoff of the drum.

The inventors have found that this problem can be avoided by flowinggases towards the nip where the web is unwound from the drum. When thegases flow towards the web under relatively low pressures, these gasesform a plasma. The plasma provides a path for the static charge on theweb to dissipate.

This plasma is found in the winding zone, and faces the back of theplastic web material. This plasma is to be distinguished from theprocess plasma that faces the front of the plastic web material ontowhich the deposited layer is placed.

The gases which are used to form the plasma can be released into abaffle in the winding zone. The gases flow into the baffle creating ahigh enough pressure to create a plasma. Then the plasma-charged gasesflow from the baffle toward the point where the web separates from thedrum. The formed plasma allows for the dissipation of the static chargeon the web. The baffle can maintain a pressure differential in thewinding zone. The pressure around the baffle and near the point wherethe web separates from the drum is greater than the pressure in theremaining portion of the winding zone. This allows for diffusion pumpsto be used in the winding zone. Additionally, the power used by theplasma in the winding zone can be kept relatively low.

In one embodiment, a baffle that is triangular-shaped in cross-sectionis used near the location where the web separates from the drum.

In a preferred embodiment, the winding zone and the deposition zone aremaintained separately so that the monomer does not enter the windingzone from the deposition zone. In this way, a dielectric layer will notbe deposited on the drum electrode.

In a preferred embodiment, the gases used for the plasma in the windingzone will be a subset of the gases used in the deposition zone. Inparticular, it has been found that a pure oxygen gas or a mixture ofoxygen and argon will work particularly well in avoiding the websticking problem. In a preferred embodiment, around twenty to thirtymicrons of gas pressure are supplied to the baffle. This compares toaround one to two microns of gas pressure in the remainder of thewinding zone and about fifty to a hundred microns of pressure in thedeposition zone.

Although maintaining a pressure differential between a baffle and theremaining portion of the winding zone is desirable when using diffusionpumps, it is not necessary for this pressure differential to exist inorder to reduce web sticking. A uniform pressure could be formed in thewinding zone to prevent web sticking. In this alternate embodiment, forexample, about twenty to thirty microns of gas pressure can be suppliedto the entire winding zone. In this manner, a plasma can be formed toavoid the web sticking.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a plasma-enhanced chemical vapor depositionsystem using a triangular-shaped baffle in the winding zone.

FIG. 2 is a diagram of a plasma-enhanced chemical vapor depositionsystem using a larger baffle in the winding zone.

FIG. 3 is a perspective view of the triangular-shaped baffle of FIG. 1.

FIG. 4 is a cross-sectional view of the triangular-shaped baffle ofFIGS. 1 and 3.

FIG. 5 is a strip-chart diagram illustrating the power, voltage, andcurrent supplied to the drum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagram of the plasma-enhanced chemical vapor depositionsystem 10 using a triangular-shaped baffle 12. In the plasma-enhancedchemical vapor deposition system 10, a metal rotatable drum 14 isconnected to the AC power supply 16 so that the metal drum 14 acts as anelectrode for the plasma deposition system. A web material 18 rolls overthe drum 14 and is guided by rollers 20.

The plasma deposition system is divided into two zones--the winding zone22 and the deposition zone 24. In the deposition zone 24, a layer suchas a silicon oxide layer is formed on the plastic web 18. In theplasma-enhanced chemical vapor deposition, a plasma is formed betweenthe metallic drum and a counter-electrode (in this case the chamberwall) of the evacuated chamber 26. The plasma causes the dissociation ofthe monomer and results in the deposition of the coating on the web 18.Gas supply 28 supplies the gases for the deposition process. In thepreferred embodiment, oxygen, helium and the vaporized monomer aresupplied into the deposition zone 24 by gas supply 28.

The baffles 19 in connection with the web 18 help divide the system intothe winding zone 22 and the deposition zone 24. It is desired that themonomer not be leaked into the winding zone 22 so that the drum 14 isnot coated with dielectric material.

The plastic web 18 rolls continuously on the drum 14 so that differentareas on the web 18 are coated with the deposited layer. Not shown inthis figure are the baffles and magnets in the deposition zone 24 usedfor shaping the plasma around the web 18 to improve the deposition. Gassupply 30 provides the gases to the baffle 12. In the preferredembodiment, these gases comprise pure oxygen or an argon/oxygen mix. Theuse of pure argon was found to be less desirable.

The gases supplied through the baffle are directed to the point A wherethe web 18 separates from the drum 14. The use of a baffle helpsmaintain the pressure differential in the winding zone 22. In apreferred embodiment, the gas pressure in the baffle area is abouttwenty to thirty microns. The pressure in the remainder of the windingzone 22 is preferably less than about five microns. In a preferredembodiment, this gas pressure is preferably one to two microns. Thiscompares to the fifty to one hundred microns of pressure that istypically found in the deposition zone 24. Maintaining two differentpressures in the winding zone 22 helps the diffusion pump 32 to operateefficiently.

FIGS. 3 and 4 show a more detailed description of the baffle 12 used inFIG. 1. FIG. 3 is a perspective view of the triangular-shaped baffleshown in FIG. 1. This baffle includes an L-shaped metal structure 40which is connected by a brace bar 42 to the ground. In the preferredembodiment, there are six gas injector ports which are connected by aplumbed line 46 to the gas supply (not shown). The baffle 12 alsoincludes plastic parts 48, 50 and 52. Note that part 52 can be curved soas to follow the shape of the drum (not shown).

FIG. 4 is a cross-sectional view of the triangular-shaped baffle 12shown in FIGS. 3 and 1. The gases flow through the port 44 form a plasmain the baffle. In the preferred embodiment, the distance d_(b) betweenthe bottom of the baffle and the drum is about 3/16"; the distance d_(t)between the top of the baffle and the drum is about 1/64". FIG. 2 is adiagram of a plasma-enhanced chemical vapor deposition apparatus 10'using a larger baffle 60. This larger baffle 60 can be extended almostfrom one side of the web 18' to the other.

An alternative to using the baffle for supplying the gases to the web18' as it separates from the drum 14' can include the use of a hollowanode that produces a plasma as gases flow through a hollow metal tube.The hollow metal tube would be grounded so that a plasma would be formedbetween the hollow anode and the drum electrode.

FIG. 5 is a diagram illustrating the power 60, voltage 62, and current64 supplied to the drum. The power 60 is at about 27 kilowatts, thevoltage 62 is at about 532 volts and the current 64 is at about 51.7amps. Until time I, gases are supplied to the baffle as described above.There are drop-outs in the power supply due to arcing, but these occurrelatively infrequently. At time I, the gas supply to the baffle isturned off. Notice that the number of power supply drop-outs increasesdramatically. At time II, the power is turned off. As discussed above,power supply dropouts can cause a reduction in the quality of the coat.

Various details of the implementation and method are merely illustrativeof the invention. It will be understood that various changes of thesedetails may be within the scope of the invention, which is to be limitedonly by the appended claims.

What is claimed is:
 1. An apparatus comprising:an evacuable chamber; arotatable drum adapted to roll a web, wherein when the web is rolledover the drum, the web divides the evacuable chamber into a depositionzone and a winding zone; the deposition zone facing the side of the webonto which a layer is to be deposited, the winding zone facing the otherside of the web; and means for forming a plasma in the winding zone, theplasma adapted to allow for the dissipation of static electricity fromthe web so that the sticking of the web to the drum is reduced.
 2. Theapparatus of claim 1, wherein the deposition zone includes means fordepositing a layer on the web by plasma-enhanced chemical vapordeposition.
 3. The apparatus of claim 1, wherein the drum is made of anelectrically conducting material and an alternating current power sourceis electrically connected to the drum.
 4. The apparatus of claim 1,wherein the plasma forming means includes an apparatus to supply gasesthat flow toward the point where the web separates from the drum.
 5. Theapparatus of claim 1, wherein the plasma forming means includes a baffleto direct the plasma toward the point where the web separates from thedrum.
 6. The apparatus of claim 5, wherein the baffle is positioned nearthe point the web is unwound from the drum.
 7. The apparatus of claim 5,wherein the baffle extends from near the point that the web is wound onthe drum to near the point the web is unwound from the drum.
 8. Theapparatus of claim 5, wherein the baffle is supported by a groundedmetal part.
 9. The apparatus of claim 8, wherein a dielectric part islocated between the metal part and the drum.
 10. The apparatus of claim5, wherein a greater gas pressure is maintained in the baffle than inthe remainder of the winding zone.
 11. The apparatus of claim 1, whereina uniform gas pressure is maintained in the winding zone.
 12. Theapparatus of claim 1, wherein when the web is in the evacuated chamber,gases are prevented from passing between the deposition zone and thewinding zone.